[x86] Hoist the logic for extracting the relevant bits of information

from the MachineInstr into the caller which is already doing a switch
over the instruction.

This will make it more clear how to compute different operands to feed
the comment selection for example.

Also, in a drive-by-fix, don't append an empty comment string (which is
a no-op ultimately).

No functionality changed.

llvm-svn: 218361